D-Mannose Side Effects: What Studies Report and Who Should Avoid It

D-mannose side effects in human trials were uncommon and mostly mild, and the one reported most often was diarrhea. No serious side effect has been linked to D-mannose in those trials, but the safety record is thin, and people who are pregnant, managing blood sugar or living with kidney disease should check with a physician before taking it. For the amounts those trials used, see our guide to D-mannose dosage.

D-mannose side effects reported in human studies

D-mannose has been tested mostly in women with repeat urinary tract infections, usually at 2 grams a day for up to six months. The studies tracked safety in different ways, as the table shows.

The pattern is consistent: when side effects were noted, they were mild and mostly digestive. But the Cochrane authors called the reporting poor, most trials were small, and none in that review lasted longer than six months.

The MERIT numbers need context. A serious adverse event is any serious health event during a trial, such as a hospital stay, whatever its cause. The investigators judged none of them to be related to D-mannose and listed serious adverse events among the outcomes that did not differ between the groups.

Is D-mannose safe?

For healthy adults at the amounts studied, the trials found D-mannose well tolerated, and a 2014 review of mannose metabolism noted no reports of harm from mannose in people at that time. But that reflects mostly healthy women taking about 2 grams a day for a few months, and says little about pregnancy, diabetes, kidney disease, children or use over many years.

Safety is also only half the question. In MERIT, the largest placebo-controlled trial, 51.0% of women on D-mannose and 55.7% on placebo needed care for another suspected urinary infection, a difference that was not statistically significant.

D-mannose diarrhea and stomach upset

Diarrhea is the side effect that turns up most often in D-mannose reports. In the 2014 trial it affected 8 of 103 women taking 2 grams a day, about 8%, and no one had to stop. The amount seems to matter: the 2014 mannose review reports that single doses below 0.2 grams per kilogram of body weight, about 13.6 grams for a 150 pound adult, did not change blood glucose, while higher doses caused mild stomach and gut discomfort.

If you notice loose stools, make sure you are not taking two products that both contain D-mannose, follow the label, and call your physician if the diarrhea does not settle or comes with blood, fever or signs of dehydration.

Can you take too much D-mannose?

No official upper limit has been set. The urinary trials used 2 to 3 grams a day, and there is no evidence that more adds anything. In a 1997 study of healthy volunteers and patients with a rare inherited disorder of sugar processing, 0.1 grams per kilogram every three hours raised blood mannose three to fivefold, with no side effects seen on that schedule. People with one such disorder take far larger amounts under medical care, and the 2014 review notes that their physicians watch for complications like those seen in diabetes. For a supplement user, the realistic downside of exceeding the label is stomach upset, with no evidence of extra benefit.

D-mannose and blood sugar: is it safe with diabetes?

D-mannose is a sugar, a close chemical relative of glucose. It occurs in small amounts in fruits such as oranges, apples and peaches, and inside cells most of it is broken down for energy much as glucose is. For more on its path through the body, see how D-mannose works.

• Does it raise blood glucose? The 2014 review reports that single doses below 0.2 grams per kilogram raised blood mannose about threefold without changing blood glucose. Supplement amounts are far below that.
• Is it a concern with diabetes? We could not find a published trial of D-mannose supplements in people with diabetes. The same review notes that mannose sticks to proteins, a process called glycation, about five times as readily as glucose, and raises the possibility of diabetes-type complications at very large therapeutic amounts. Whether supplement amounts matter here is unknown.

If you have diabetes or prediabetes, or take medicine that lowers blood sugar, talk with your physician first and watch your readings when you add any new supplement.

D-mannose in pregnancy and breastfeeding

There is no good human safety data for D-mannose in pregnancy or breastfeeding, and MERIT excluded women who were pregnant, breastfeeding or planning pregnancy. The main caution comes from animal work. The 2014 review describes mouse studies in a strain with an inherited defect in an enzyme that breaks mannose down. When pregnant mice of that strain were given extra mannose, embryos died at higher amounts, and about half of the pups born at lower amounts developed eye defects. Ordinary mice given mannose during pregnancy had normal litters. The authors said it is unknown whether this applies to people, but because no one knows how common such defects are in humans, they urged caution in pregnancy. Until there is better evidence, avoid D-mannose while pregnant or breastfeeding unless your physician advises otherwise.

D-mannose and kidneys

D-mannose is absorbed in the upper gut and passed out in the urine, which is why it has been studied for the urinary tract. None of the trials above reported kidney problems linked to it, but we could not find a published trial in people with chronic kidney disease, a group the 2022 Cochrane review lists among those at greater risk of urinary infections. Reduced kidney function slows the clearance of many substances, so anyone with kidney disease or a kidney transplant should ask their physician first.

D-mannose and cranberry interactions

Many D-mannose products also contain cranberry. We did not find published reports of drug interactions with D-mannose itself; the better-documented cautions involve cranberry.

• Warfarin. A 2012 case report and literature review described an 85-year-old woman on warfarin whose INR, a blood clotting test, rose two to threefold after cranberry sauce on two occasions. Most controlled studies found no significant interaction; in one from 2010, 10 men on warfarin drank cranberry juice twice a day for a week with no change in clotting time. If you take a blood thinner, ask your physician or pharmacist before adding cranberry.
• Oxalate and kidney stones. Oxalate is a common part of kidney stones. In a small 2001 study, 5 healthy volunteers took cranberry concentrate tablets for a week and the oxalate in their urine rose by an average of 43%. Studies of cranberry juice have pointed in different directions. If you have had kidney stones, ask your physician before taking a cranberry product.

For how the two ingredients are paired, see D-mannose and cranberry together.

Should you take more D-mannose for urinary symptoms?

No. Burning, urgency, cloudy or bloody urine, fever, or back or side pain can mean an infection that needs testing and often a prescription, and an infection that is not properly managed can spread to the kidneys. The 2022 Cochrane review found little to no evidence either way for D-mannose once an infection has started. In MERIT, women kept taking the study product while they had symptoms yet needed care about as often as those on placebo. See a physician promptly, and seek urgent care for fever, vomiting, or back or side pain.

Who should check with a physician first

• Anyone with urinary symptoms now, such as burning, urgency, fever, back or side pain, or blood in the urine.
• Anyone pregnant, breastfeeding or planning pregnancy.
• Anyone with diabetes or prediabetes, or who takes medicine that lowers blood sugar.
• Anyone with kidney disease or a kidney transplant.
• Anyone on warfarin or another blood thinner, or with a history of kidney stones, who is considering a product that also contains cranberry.
• Men and children with urinary symptoms. The main trials enrolled adult women.
• Anyone taking prescription medication. Ask your pharmacist to review the full label.

How Vita Bloom fits

UroDefend is our UroDefend D-mannose and cranberry supplement, with pine bark extract and vitamin C, in one daily two-capsule serving. Each serving provides 300 mg D-mannose, 500 mg standardized cranberry extract (12:1), 100 mg pine bark extract and 100 mg vitamin C, to support urinary tract health. The label directs two capsules daily with food and says to ask your clinician first if you are pregnant, breastfeeding or taking medication. UroDefend is a daily supplement for everyday wellness; new or worsening urinary symptoms need a physician.

Frequently asked questions

What are the most common D-mannose side effects?

Diarrhea is reported most often, and the 2022 Cochrane review also found reports of vaginal burning. In trials at about 2 grams a day, side effects were uncommon and mild, and no serious side effect was linked to D-mannose.

Does D-mannose cause diarrhea?

It can. In the 2014 trial, 8 of 103 women taking 2 grams a day had diarrhea. If it does not settle, stop and check with your physician.

Does D-mannose raise blood sugar?

A 2014 review reports that single doses below 0.2 grams per kilogram of body weight did not change blood glucose. We found no trial in people with diabetes, so check with your physician if you manage blood sugar.

Is D-mannose safe during pregnancy?

That is not known. Pregnant women were excluded from the largest trial, and a 2014 review urged caution based on mouse studies in a strain with an inherited enzyme defect. Avoid it unless your physician advises otherwise.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.